FINAL DRAFT
ECAO-CIN-D023
United States JU|y 1993
Environmental Protection '
Agency
v°/EPA Research and
Development
REVISED AND UPDATED DRINKING WATER
QUANTIFICATION OF TOXICOLOGIC EFFECTS
FOR METHYL TERT-BUTYL ETHER (MTBE)
Prepared for
Office of Water
Prepared by
Environmental Criteria and Assessment Office
Office of Health and Environmental Assessment
U.S. Environmental Protection Agency
Cincinnati, OH 45268
-------
DISCLAIMER
This report is an external draft for review purposes only and does not constitute
Agency policy. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
-------
PREFACE
Section 1412 (b)(3)(A) of the Safe Drinking Water Act, as amended in 1986, requires
the Administrator of the U.S. Environmental Protection Agency to publish maximum
contaminant level goals (MCLGs) and promulgate National Primary Drinking Water
Regulations for each contaminant, which, in the judgment of the Administrator, may have
an adverse effect on public health and that is known or anticipated to occur in public
water systems. The MCLG is nonenforceable and is set at a level at which no known or
anticipated adverse health effects in humans occur and that allows for an adequate
margin of safety. Factors considered in setting the MCLG include health effects data and
sources of exposure other than drinking water.
This document provides the health effects basis to be considered in establishing the
MCLG. To achieve this objective, data on toxicokinetics and acute, subchronic and
chronic toxicrty to animals and humans are evaluated. Specific emphasis is placed on
data published in peer-reviewed literature providing dose-response information. Thus,
while the literature search and evaluation performed in the development of this document
have been comprehensive, only the reports considered most pertinent in the derivation
of the MCLG are cited in the document The comprehensive literature data base in
support of this document includes information published up to 1992; however, more
recent data may have been added during the review process.
When adequate health effects data exist, Health Advisory (HA) values for less-than-
lifetime exposures (1-day, 10-day and longer-term, i.e., -10% of an individual's lifetime)
are included in this document These values are not used in setting the MCLG, but serve
as informal guidance to municipalities and other organizations when emergency spills or
contamination situations occur. With adequate data, a Reference Dose (RfD) is derived
to be utilized in the derivation of a Drinking Water Equivalent Level (DWEL) on which the
MCLG is based. Also provided is the U.S. EPA's determination of the contaminant's
carcinogenic potential. When the contaminant has been determined to be a probable or
possible human carcinogen, the estimated excess cancer risk associated with ingestion
of contaminated water is included.
This document was prepared for the Office of Water by the Office of Health and
Environmental Assessment (Environmental Criteria and Assessment Office, Cincinnati,
Ohio) to provide the scientific support for the human health-based risk assessment used
in the determination of the drinking water MCLG. For more information, contact the
Human Risk Assessment Branch of the Office of Water at (202)260-7571.
in
-------
TABLE OF CONTENTS
Pace
INTRODUCTION 1
. •
BACKGROUND 1
PHYSICAL AND CHEMICAL PROPERTIES, USES AND OCCURRENCE 7
TOXICOKINETICS 8
NONCARCINOGENIC EFFECTS IN HUMANS 13
NONCARCINOGENIC EFFECTS IN LABORATORY ANIMALS 13
SHORT-TERM EXPOSURE 13
LONG-TERM EXPOSURE 19
REPRODUCTIVE AND DEVELOPMENTAL TOXICITY 27
QUANTIFICATION OF NONCARCINOGENIC EFFECTS 32
CARCINOGENIC EFFECTS 32
HUMAN DATA , 32
LABORATORY ANIMAL DATA . 32
Oral Studies 32
Inhalation Studies 33
Injection Studies 35
SHORT-TERM STUDIES 36
QUANTIFICATION OF CARCINOGENIC EFFECTS 37
WEIGHT-OF-EVIDENCE FOR CLASSIFICATION -. 37
QUANTITATIVE ESTIMATE 37
EXISTING GUIDELINES, RECOMMENDATIONS AND STANDARDS 37
SPECIAL GROUPS AT RISK 37
REFERENCES 38
IV
-------
LIST OF ABBREVIATIONS
AST Aspartate aminotransferase
AUC Area under curve
BUN Blood urea nitrogen • •
CAS Chemical Abstract Service
CMS Central nervous system
OWEL Drinking water equivalent level
PEL Frank effect level
HA Health advisory
HCT Hemocrit
LOH Lactate dehydrogenase
LOAEL Lowest-observed-adverse-effect level
LOEL Lowest-observed-effect level
MCH Mean corpuscular hemoglobin
MCHC Mean corpuscular hemoglobin concentration
MCV Mean corpuscular volume
MTD Maximum tolerated dose
NOAEL No-observed-adverse-effect level
RBC Red blood cell
RfD Reference dose
-------
LIST OF ABBREVIATIONS
AST Aspartate aminotransferase
AUC Area under curve
BUN Blood urea nitrogen
CAS Chemical Abstract Service
CMS Central nervous system
DWEL Drinking water equivalent level
FEL Frank effect level
HA Health advisory
HCT Hemocrit
LDH Lactate dehydrogenase
LOAEL Lowest-observed-adverse-effect level
LOEL Lowest-observed-effect level
MCH Mean corpuscular hemoglobin
MCHC Mean corpuscular hemoglobin concentration
MCV Mean corpuscular volume
«
MTD Maximum tolerated dose
NOAEL No-observed-adverse-effect level
RBC Red blood cell
RfD Reference dose
VI
-------
INTRODUCTION
BACKGROUND
The quantification of toxicologic effects of a chemical consists of separate
assessments of noncarcinogenic and carcinogenic health effects. Chemicals that do not
produce carcinogenic effects are believed to have a threshold dose below which no
adverse, noncarcinogenic health effects occur, while carcinogens are assumed to act
without a threshold.
In the quantification of noncarcinogenic effects, a Reference Dose (RfD), [formerly
termed the Acceptable Daily Intake (ADI)] is calculated. The RfD is an estimate (with
uncertainty spanning perhaps an order of magnitude) of a daily exposure to the human
population (including sensitive subgroups) that is likely to be without an appreciable risk
of deleterious health effects during a lifetime. The RfD is derived from a no-observed-
adverse-effect level (NOAEL), or lowest-observed-adverse-effect level (LOAEL), identified
from a subchronic or chronic study, and divided by an uncertainty factor(s) times a
modifying factor. The RfD is calculated as follows:
RfD (NOAEL or LOAEL) _ mg/kg bw/day
[Uncertainty Factor(s) x Modifying Factor]
Selection of the uncertainty factor to be employed in the calculation of the RfD is
based upon professional judgment, while considering the entire data base of toxicologic
MTBE.QTE -1- 09/28/92
-------
effects for the chemical, in order to ensure that uncertainty factors are selected and
applied in a consistent manner, the U.S. EPA (1993) employs a modification to the
guidelines proposed by the NAS (1977, 1980) as follows:
•
Standard Uncertainty Factors (UFs)
• Use a 10-fold factor when extrapolating from valid experimental results from
studies using prolonged exposure to average healthy humans. This factor
is intended to account for the variation in sensitivity among the members
of the human population. [10H]
• Use an additional 10-fold factor when extrapolating from valid results of
long-term studies on experimental animals when results of studies of
human exposure are not available or are inadequate. This factor is
intended to account for the uncertainty in extrapolating animal data to the
case of humans. [10A]
• Use an additional 10-fold factor when extrapolating from less than chronic
results on experimental animals when there are no useful long-term human
data. This factor is intended to account for the uncertainty in extrapolating
from less than chronic NOAELs to chronic NOAELs. [10S]
• Use an additional 10-fold factor when deriving an RfD from a LOAEL
instead of a NOAEL This factor is intended to account for the uncertainty
in extrapolating from LOAELs to NOAELs. [10L]
Modifying Factor (MF)
• Use professional judgment to determine another uncertainty factor (MF)
that is greater than zero and less than or equal to 10. The magnitude of
the MF depends upon the professional assessment of scientific uncer-
tainties of the study and data base not explicitly treated above, e.g., the
completeness of the overall data base and the number of species tested.
The default value for the MF is 1.
MTBE.QTE -2- 01/20/93
-------
The uncertainty factor used for a specific risk assessment is based principally upon
scientific judgment rather than scientific fact and accounts for possible intra- and
interspecies differences. Additional considerations not incorporated in the NAS/ODW
guidelines for selection of an uncertainty factor include the use of a less-than-lifetime
study for deriving an RfO, the significance of the adverse health effects and the counter-
balancing of beneficial effects.
From the RfD, a Drinking Water Equivalent Level (DWEL) can be calculated. The
DWEL represents a medium specific (i.e., drinking water) lifetime exposure at which
adverse, noncarcinogenic health effects are not anticipated to occur. The OWEL assumes
100% exposure from drinking water. The DWEL provides the noncarcinogenic health
effects basis for establishing a drinking water standard. For ingestion data, the DWEL is
derived as follows:
DWEL m (fflD) x (Body weight in kg) m /L
Drinking Water Volume In L/day
where:
Body weight = assumed to be 70 kg for an adult
Drinking water volume = assumed to be 2 L/day for an adult
MTBE.QTE -3- 01/20/93
-------
In addition to the RfD and the DWEL, Health Advisories (HAs) for exposures of
shorter duration (1-day, 10-day and longer-term) are determined. The HA values are
used as informal guidance to municipalities and other organizations when emergency
spills or contamination situations occur. The HAs are calculated using an equation similar
to the RfD and DWEL; however, the NOAELs or LOAELs are identified from acute or
subchronic studies. The HAs are derived as follows:
HA - (NOAEL <" LOAEL) * W -
"* (UF) x (_ L/C*) —
Using the above equation, the following drinking water HAs are developed for
noncartinogenic effects:
1. 1-day HA for a 10 kg child ingesting 1 L water per day.
2. 10-day HA for a 10 kg child ingesting 1 L water per day.
3. Longer-term HA for a 10 kg child ingesting 1 L water per day.
4. Longer-term HA for a 70 kg adult ingesting 2 L water per day.
The 1-day HA calculated for a 10 kg child assumes a single acute exposure to the
chemical and is generally derived from a study of <7 days duration. The 10-day HA
assumes a limited exposure period of 1-2 weeks and is generally derived from a study
of <30 days duration. The longer-term HA is derived for both the 10 kg child and a 70
kg adult and assumes an exposure period of -7 years (or 10% of an individual's lifetime).
MTBE.QTE -4- 09/28/92
-------
The longer-term HA is generally derived from a study of subchronic duration (exposure
for 10% of animal's lifetime).
The U.S. EPA categorizes the carcinogenic potential of a chemical, based on the
overall weight-of-evidence, according to the 'following scheme:
Group A: Human Carcinogen. Sufficient evidence exists from epidemiology
studies to support a causal association between exposure to the chemical and
human cancer.
Group B: Probable Human Carcinogen. Sufficient evidence of cardnogenicrty
in animals with limited (Group B1) or inadequate (Group B2) evidence in
humans.
Group C: Possible Human Carcinogen. Limited evidence of cardnogenicrty
in animals in the absence of human data.
Group D: Not Classified as to Human Carcinogenicity. Inadequate human and
animal evidence of Carcinogenicity or for which no data are available.
Group E: Evidence of Noncarcinogenicity for Humans. No evidence of
Carcinogenicity in at least two adequate animal tests in different species or in
both adequate epidemiotogic and animal studies.
If toxicologic evidence leads to the classification of the contaminant as a known,
probable or possible human carcinogen, mathematical models are used to calculate the
estimated excess cancer risk associated with the ingestion of the contaminant in drinking
water. The data used in these estimates usually come from lifetime exposure studies
using animals. In order to predict the risk for humans from animal data, the animal dose-
response is converted, using experimentally derived or default assumptions, to equivalent
MTBE.QTE -5- 09/28/92
-------
human values. The conversion includes correction for noncontinuous exposure, less-
than-lifetime exposure and differences in pharmacologic effects. The default factor that
compensates for the differences in pharmacologic effects is related to body surface area,
which is proportional to body weight to the two-thirds power. It is assumed that the
average adult human body weight is 70 kg and that the average water consumption of
an adult human is 2 L water per day.
For contaminants with a carcinogenic potential, chemical levels are correlated with
a carcinogenic risk estimate by employing a cancer unit risk (sometimes referred to as
a cancer potency) value together with the assumption for lifetime exposure from ingestion
of water. The cancer unit risk is usually derived from a linearized multistage model with
a 95% upper confidence limit, thus providing a low dose upper bound estimate of risk;
that is, the true risk to humans, while not identifiable, is not likely to exceed the upper
»
bound estimate and, in fact, may be lower. Excess cancer risk estimates may also be
calculated using other models such as the one-hit, Weibull, logrt and probit There is little
basis in the current understanding of the biologic mechanisms involved in cancer to
suggest that any one of these models is able to predict the true risk more accurately than
any other. Because each model is based upon differing assumptions, the estimates
derived for each model can differ by several orders of magnitude.
The scientific data base used to calculate and support the setting of cancer risk rate
levels has an inherent uncertainty that is due to the systematic and random errors in
MTBE.QTE -6- 09/28/92
-------
scientific measurement. In most cases, only studies using experimental animals have
been performed. Thus, there is uncertainty when the data are extrapolated to humans.
When developing a cancer risk characterization, several other areas of uncertainty exist,
such as the usual incomplete knowledge about interrelated health effects of contaminants
• •
in drinking water, the impact on the observed response of the experimental animal's age,
sex and species compared with humans, and the actual dose received by the internal
organs in experimental animals or expected in humans. Dose-response data usually are
available only for high levels of exposure and not for the lower levels of exposure closer
to where a standard may be set. When there is exposure to more than one contaminant,
additional uncertainty results from a lack of information about possible synergistic or
antagonistic effects.
PHYSICAL AND CHEMICAL PROPERTIES, USES AND OCCURRENCE
The saturated aliphatic ether methyl tertiary- butyl ether, commonly called MTBE, is
known as 2-methoxy-2-methylpropane by the Chemical Abstracts Service (CAS). The
CAS Registry number for MTBE is 1634-04-4. It has a molecular formula of C5H12O and
a molecular weight of 88.15. MTBE is a colorless liquid that freezes at -109°C and boils
at 55.2°C (Weast, 1985). It has a density of 0.74 at 20°C (Weast, 1985) and a water
\
solubility of 51,260 mg/L at 25°C (Yalkowsky, 1989). It is soluble in ethanol and ethyl
ether (Weast, 1985). The vapor pressure of MTBE is 249.3 mm Hg at 25°C (Daubert and
Danner, 1992). It is used as an octane booster for unleaded gasoline, in the manufacture
MTBE.QTE -7- 05/06/93
-------
of isobutene and recently in the non-surgical treatment of gallbladders (Hawley, 1981;
Bruckstein, 1990).
The annual mean concentration of MTBE in the atmosphere in the United States
during 1987-1988 was <0.2 ppb (v/v) (LaGrbne, 1991). Potable water from a few private
wells from the state of Connecticut contained MTBE in the concentration range of 1-7750
(ig/L (U.S. EPA, 1987). The concentration of MTBE in potable well water in Raynham,
MA, ranged from not detected to 22.0 u.g/L, with a mean value of 7.8 u,g/L (U.S. EPA,
1987). Three percent of potable well water samples in the state of Maine contained
MTBE in the concentration range of 20-236,000 u,g/L (U.S. EPA, 1987). Potable water
from two private wells in New Mexico contained 70 and 350 ng/L of MTBE (U.S. EPA,
1987). In New Hampshire, MTBE was detected at concentrations ranging from not
detected to 100 (ig/L in 145 potable well water samples, at concentrations ranging from
101-1000 ^g/L in 20 samples and at concentrations ranging from 1001-10,000 u.g/L in 11
samples (U.S. EPA, 1987).
TOXICOKINETICS
The toxicokinetics of MTBE (purity 99%) were evaluated in rats after oral administra-
tion by Bioresearch Laboratories (1990a). Groups of 10, 40 and 40 Fischer 344 rats
(59-61 days old, fasted) of each sex were administered single doses of 0, 40 or 400
mg/kg, respectively, by gavage in saline vehicle. Plasma concentrations of MTBE and
t-butanol (a major circulating metabolite) were determined in four males and four females
in each treated group at various times up to 36 hours after dosing. Area under curve
MTBE.QTE -8- 08/02/93
-------
in each treated group at various times up to 36 hours after dosing. Area under curve
(AUC) comparisons showed that levels of MTBE and t-butanol were similar in both sexes,
indicating no gender-related pharmacokinetic differences. Gastrointestinal absorption of
MTBE was rapid as shown by maximum plasma concentration 15 minutes post-exposure
(the first sampling time) at both low and high doses. t-Butanol was rapidly formed with
peak plasma concentrations occurring 2 hours post-exposure. Relative to the increase
in dose (from 40-400 mg/kg), the high dose of MTBE gave a greater than proportional
increase in MTBE AUC and a less than proportional increase in t-butanol AUC,
suggesting a saturation of the enzymes catalyzing the formation of t-butanol from MTBE.
The plasma elimination half-life of MTBE, based on a one compartment model for the
plasma concentration/time curve, was approximately 0.45-0.62 and 0.79-0.93 hours after
the low and high doses, respectively. The plasma elimination half-life of t-butanol was
approximately twice as long as that of MTBE.
•
In a related mass balance and metabolism experiment, groups of six male and six
female Rscher 344 rats (fasted) were administered single 0, 40 or 400 mg/kg doses of
14C-MTBE in saline vehicle by gavage (Bioresearch Laboratories, 1990b). Urine, feces
and expired air were collected at various times £48 hours post-treatment for determination
of 14C-MTBE, 14CO2 and other 14C-metabolites (e.g., t-butanol and acetone). There was
no further collection of excreta and expired air because, based on the amount of
radioactivity remaining in the tissues and carcasses at 48 hours (<2% of the dose), it was
unlikely that overall recoveries would have significantly increased.
MTBE.QTE -9- 05/06/93
-------
The total recovery of radioactivity after 48 hours was 85.2±2.9% and 86.3±5.5% of
the 40 mg/kg dose in the males and.females, respectively, and 81.8±6.3% and
80.2±10.7% of the 400 mg/kg dose in the males and females, respectively. The two
major routes of excretion were the lungs and kidneys. At 40 mg/kg, the mean recovery
of radioactivity was 36.2% (males) and 29.0% (females) in the urine and 45.8% (males)
and 54.4% (females) in the expired air. At 400 mg/kg, a larger proportion of the dose
was exhaled from the lungs [65.3% (males) and 68.7% (females)] than at the lower dose,
possibly due to saturation of MTBE metabolizing enzymes. At the high dose, a lower
proportion of the dose was eliminated by urinary excretion [16.0% (males) and 10.8%
(females)]. Approximately 55.6% and 59.2% of the 400 mg/kg dose was exhaled by the
males and females, respectively, within the first 3 hours after dosing. The clearance from
the lungs was thought to be a function of the blood/air partition coefficient of MTBE. The
rats in the 400 mg/kg dose group having the highest total radioactivity recoveries (88-92%
of the dose) were those with the highest recoveries in the expired air. Therefore, it is
likely that the relatively lower total recoveries at 400 mg/kg (compared to the low dose),
and incomplete recoveries of radioactivity at both doses, was due to a rapid exhalation
of MTBE between treatment and the start of expired air collection. The amount of
radioactivity recovered in the feces of both sexes was -1% of the low dose and 0.3% of
the high dose. The amount of radioactivity combined in the tissues and carcass at 48
hours post-treatment ranged from 1.94-2.00% of the low dose and 0.67-1.02% of the high
dose. Therefore, although total recoveries were incomplete, the low recoveries of
MTBE.QTE -10- 07/08/93
-------
radioactivity in the feces, tissues and carcass and other data suggest that the absorption
and elimination of MTBE were rapid and virtually complete.
The major biotransformation pathway appeared to be oxidative: demethylation to
t-butanol, oxidation of t-butanol to 2-methyl-1,2-propanediol, and further oxidation to
a-hydroxyisobutyric acid (Bioresearch Laboratories, 1990b). The radioactivity in urine was
composed mainly of 2-methyl-1,2-propanediol and a-hydroxyisobutyric acid. Pilot studies
indicated that glucuronide conjugation was not a significant pathway for any of these
metabolites. Most of the radioactivity in the expired air was present as MTBE with a
small amount (-1-3%) occurring as t-butanol.
The toxicokinetics of MTBE following inhalation exposure was studied by Savolainen
et al. (1985). In this study, male Wistar rats were exposed to 0, 50, 100 or 300 ppm
MTBE 6 hours/day, 5 days/week for 2, 6, 10 or 15 weeks. MTBE and t-butanol, a
metabolite of MTBE, were detected in the blood, brain and perirenal fat of the animals at
all time points, indicating that MTBE was absorbed from the lungs, distributed systemically
and metabolized to t-butanol. The levels of MTBE and t-butanol in the blood were related
to the levels of inhaled MTBE; the level of MTBE in the blood tended to decrease slightly
(at 50 ppm) or remain fairly constant (at higher exposures) after 6 weeks of exposure,
while the level of t-butanol increased considerably at all exposure levels after 6 weeks of
exposure, and subsequently decreased. Levels of MTBE and t-butanol in the brain
showed a similar pattern. Only MTBE was detected in the perirenal fat and the levels
tended to decrease following 2 weeks of exposure. Exposure resulted in a transient but
MTBE.QTE -11- 07/08/93
-------
significant dose-dependent increase in UDP-gluconosyrtransferase in liver and kidney
microsomes and a slight increase in renal, but not hepatic, cytochrome P-450 activity.
The toxicokinetics of MTBE following intraperitoneal administration were studied by
Bio/dynamics, Inc. (1984). Charles River CD rats were injected intraperitoneally with
"C-MTBE (60 uCi/rat, average dose of 232 mg MTBE/kg bw) and the animals were
sacrificed at 5, 15, 30 or 45 minutes, or 1, 2, 3, 6, 12, 24 or 48 hours after treatment.
Samples of blood, tissues, urine, feces and expired air were taken at various intervals of
exposure, and the radioactive content of the samples was determined. In selected
samples, the quantitation of 14C-MTBE and 14C-metabolites was also determined, along
with a total 14C-content. Peak blood levels occurred 5 minutes after injection, and plasma
levels peaked 5 and 15 minutes after injection for male and female rats, respectively.
The half-lives of MTBE in the blood were 59.8 minutes (males) and 49 minutes (females)
and in the plasma were 2.3 hours (males) and 1.3 hours (females). Blood and plasma
levels of 14C-MTBE indicated rapid absorption and elimination. Forty-eight hours after
administration, an average of 103.83% of the injected radioactive dose was recovered,
99.86% of 14C in expired air (91.75% as 14C-MTBE, 7.45% as 14CO2 and the remainder
not quantitated), 2.95% in urine and 1.02% in the feces (3.08% as formic acid and the
remainder not quantitated). Evidence of very low levels of methanol and formic acid were
found in the plasma, liver and kidneys at 15 minutes, 6 hours and 24 hours after
treatment. Radioactivity was found in the urine, feces, blood, liver, kidney and expired
MTBE.QTE -12- 07/08/93
-------
air, indicating that absorption from the peritoneum, distribution to the tissues of the body
and excretion from the body had occurred.
NONCARCINOGENIC EFFECTS IN HUMANS
MTBE has been used in humans as a cholelitholytic agent (to dissolve gallstones)
(Allen et al., 1985a,b; Gonzaga et al., 1985; Julian! et al., 1985; Wyngaarden, 1986). This
process involves perfusion of MTBE through a catheter directly into the gallbladder. Most
reports have indicated that side effects were not seen, but one case of acute renal failure
was described and attributed to hemolysis due to leakage of MTBE alongside the catheter
during a 7-hour infusion (Ponchon et al., 1988). Clinical evaluations have not been
sufficient to rule out the possibility of other acute or long-term side effects.
NONCARCINOGENIC EFFECTS IN LABORATORY ANIMALS
SHORT-TERM EXPOSURE
In an oral lethality study, six groups of five male and five female Sprague-Dawley
rats were administered MTBE by gavage at 1900,2450,3160,4080,5270 or 6810 mg/kg
bw. The rats were observed for immediate effects after dosing, at 1 and 4 hours, and
then daily for 14 days. The LD^ of MTBE was determined to be 3866 mg/kg bw with
95% confidence limits of 3327 and 4492 mg/kg (Arco Chemical Company, 1980) following
administration by gavage. At doses £4080 mg/kg, the animals experienced "marked
central nervous system (CMS) depression, ataxia, tremors, labored respiration and loss
of the righting reflex." Ataxia was observed at 2450 and 3160 mg/kg, and slight to
MTBE.QTE -13- 08/02/93
-------
marked CMS depression was observed at 1900 mg/kg, the lowest dose tested. Deaths
were observed at all but the lowest dose. A reduction or absence of clinical signs of
toxicity was observed in surviving rats at 24 hours after dosing. No grossly observable
lesions were seen in examinations of major organ systems at 1900 mg/kg; at higher
doses, the few grossly observable lesions "could be attributed to the irritating nature of
the ether.' No additional information regarding necropsy results was reported.
In a 14-day oral toxicity study, groups of 10 male and 10 female Sprague-Dawley
rats (age -10 weeks) were administered MTBE (purity £99.95%) in com oil vehicle by
gavage in doses of 0, 357, 714, 1071 and 1428 mg/kg/day for 14 consecutive days
(Robinson et al., 1990). Clinical signs (daily), body weight (Initial, days 4 and 6 and
terminal), food and water consumption (throughout study), hematology and clinical
chemistry (prior to termination) and pathology (scheduled and unscheduled deaths) we/e
evaluated in all rats. The pathology evaluations included organ weight measurements
(brain, liver, spleen, lungs, thymus, kidneys, adrenals, heart, gonads) and gross
examinations of all major organs. Histology was evaluated in the major organs of the
control and high-dose groups and in the target tissue(s) (if identified) in the remaining
dose groups.
Treated rats of both sexes in all dose groups developed loose stools by the third day
of treatment, and continuing throughout the study. Profound anesthesia occurred
immediately after dosing in the male and female rats treated with 1428 mg/kg/day.
MTBE.QTE -14- 08/02/93
-------
Normal motor and sensory functions returned within -2 hours of treatment and anesthetic
effects were not observed in other dosed or control groups. Early deaths (two males at
357 mg/kg/day, two males and two females at 1428 mg/kg/day) were attributed to gavage
errors, but these errors may have been secondary to MTBE-induced irritation of
pharyngeal mucous membranes which contributed to difficulty in passing dosing needles.
Average food consumption, water consumption, total weight gain and final body weight
were generally reduced in both sexes at the higher doses, but the only decreases that
were statistically significant (p£0.05) were decreased food intake in males at 714
mg/kg/day and females at 1428 mg/kg/day, and decreased weight gain in males at £714
mg/kg/day and females at £1071 mg/kg/day.
Red blood cells (RBC) and hemoglobin were significantly increased in males at £714
mg/kg/day. Increases in these hematologic indices in treated females were not
statistically significant. Clinical chemistry alterations generally occurred at the high dose
or inconsistently at lower doses, including significantly increased serum cholesterol in
females at 714 and 1071 mg/kg/day and males at 1428 mg/kg/day, increased serum
aspartate aminotransferase (AST) and lactate dehydrogenase (LDH) in males at £1071
mg/kg/day, decreased serum creatinine and increased serum glucose in females at 1428
mg/kg/day, and decreased blood urea nitrogen (BUN) in females and males at 1428
mg/kg/day.
MTBE.QTE -15- 08/02/93
-------
Statistically significant (p^O.05) changes in organ weight included reduced absolute
and relative lung weights in females at >357 mg/kg/day, decreased absolute and relative
thymus weights and absolute spleen weight in females at 1428 mg/kg/day, increased
relative kidney weight in males at £1071 mg/kg/day and females at 1428 mg/kg/day, and
decreased brain weight in males (absolute) and females (relative) at 1428 mg/kg/day.
Relative lung weights were also reduced in treated males, but differences were
significantly different from controls only at 714 mg/kg/day.
The only pathologic finding considered to be treatment-related was renal tubular
disease in male rats receiving 1428 mg/kg/day. The renal lesions, characterized by
increased hyaline droplets within the cytoplasm of proximal tubular epithelial cells,
occurred in seven of eight (88%) treated males and two of five (40%) controls. These
changes were consistent with male rat hyaline droplet (o^-globulin) nephropathy. No
information on kidney histology in the lower dose groups was reported.
In male Sprague-Dawley rats, the inhalation LCW for a single 4-hour exposure to
Arco MTBE (96.2% pure) was determined to be 142 mg/L (39,400 ppm) with 95%
confidence limits of 120 mg/L (33,300 ppm) and 168 mg/L (46,600 ppm). After exposure
to commercial MTBE (99.1% pure), the LCW was 120 mg/L (33,300 ppm) with 95%
confidence limits of 104 mg/L (28,900 ppm) and 139 mg/L (38,600 ppm) (Arco Chemical
Company, 1980). The concentrations tested ranged from -70 (-19,400 ppm) to -230
mg/L (-63,800 ppm). The observation period was 14 days. During this period, the
MTBE.QTE -16- 08/02/93
-------
animals experienced eye irritation, irregular respiration, ^coordination, ataxia, loss of
righting reflex and prostration. The intensity of the effects increased with increasing
exposure level.
. •
No deaths occurred in 10 New Zealand white rabbits treated with occlusive dermal
applications of 10 mg/kg of Arco MTBE or commercial MTBE, indicating that the dermal
LDgo for MTBE is >10 mg/kg bw for the rabbit (Arco Chemical Company, 1980).
Epidermal scaling and thickening were found at the site of administration.
In a repeated-exposure inhalation study, Sprague-Dawley rats (20 animals/sex/
group) were exposed to 0, 101, 300, 1020 and 2970 ppm MTBE (>98% pure) 6
hours/day, 5 days/week for 9 exposure days (Bio/dynamics, Inc., 1981) to determine
appropriate dose levels for later reproduction studies. Most of the animals were fasted
prior to sacrifice, but five animals/sex/dose were not fasted due to technician error.
Survival, urinalysis and hematologic and clinical chemistry parameters were monitored for
all of the animals. Gross necropsy and organ weight determinations were performed on
all animals; extensive histologic examinations were performed on all animals in the control
and the 2970 ppm group. The trachea, nasal turbinates, kidney and liver were examined
histologically in animals of the 1020 ppm group. No deaths, changes in "physical
observations' (including righting reflex), body weight changes or abnormalities in
urinalysis were attributed to exposure to MTBE. Clinical chemistry analyses revealed a
statistically significant (p£0.05) increase in phosphorus levels in blood from fasted female
MTBE.QTE -17- 08/02/93
-------
animals exposed to the two highest concentrations. An increase in relative liver weights
was found in the high dose groups of both the fasted males and the females, and a
similar trend was found in the high dose unfasted male animals. No compound-related
effects were found upon gross necropsy of all animals, but an increased incidence and
severity of chronic inflammation was found in the nasal mucosa and the trachea of
animals exposed to 1020 ppm (27/40) and 2970 ppm (27/40), compared with controls.
In a inhalation range-finding study (Dodd and Kintigh, 1989), Fischer 344 rats and
CD-1 mice (5/sex/species/exposu/e-level) were exposed to target levels of 0,2000, 4000
or 8000 ppm for 6 hours/day for 13 consecutive days. The animals were observed during
exposure for neurotoxic or other effects and the rats received a neurobehavioral
observational battery immediately after the 13th exposure. Hypoactivrty was observed
during exposure in both species at 2000 ppm on days 2 and 3; hypoactivtty and ataxia
were observed in both species during exposure to 4000 and 8000 ppm on most exposure
days and prostration of two of five female mice occurred at 8000 ppm on day 9. The
neurobehavioral observational battery administered to the rats immediately after the 13th
exposure, however, revealed effects only at 8000 ppm. These effects included ataxia,
decreased startle and pain reflexes, or decreased muscle tone in all of the male and most
of the female rats. The effects were reversible (i.e., not detectable 1 hour after the initial
testing). Additional significant findings included increased relative liver weights in rats at
£4000 ppm and increased relative liver weights in all MTBE-treated groups of mice. No
compound-related macroscopic lesions were detected during necropsy and no compound-
MTBE.QTE -18- 08/02/93
-------
related histplogic effects were found, but only grossly observable lesions were examined
historically.
LONG-TERM EXPOSURE
In the only subchronic oral study, groups of 10 male and 10 female Sprague-Dawley
rats (age -10 weeks) were administered MTBE (purity £99.95%) in com oil vehicle by
gavage in doses of 0, 100, 300, 900 or 1200 mg/kg/day for 90 consecutive days
(Robinson et a I., 1990). Evaluations included clinical signs (daily), body weight (initial and
biweekly) and food and water consumption (once and three times weekly, respectively).
Hematology, clinical chemistry, organ weights and pathology were evaluated as in the
14-day study described in the preceding section on short-term exposure. It is implied, but
not specifically stated, that the histology examinations were performed for the control and
high-dose groups, but were limited to the target tissue(s) in the other dose groups.
Rats in the 1200 mg/kg/day dose group exhibited profound anesthesia immediately
following treatment but recovered within 2 hours. Mortality occurred in some of the
treated males (one, two and one deaths at 100, 900 and 1200 mg/kg/day, respectively)
and females (one, two and four deaths at 300, 900 and 1200 mg/kg/day, respectively),
but was not clearly related to dose in the males and, based on pathology findings and
observations during dosing, was attributed to gavage error secondary to pharyngeal
mucous membrane irritation.
MTBE.QTE -19- 08/02/93
-------
Changes in daily average water consumption were generally inconclusive. Female
rats treated with 100 or 1200 mg/kg/day had significantly increased water consumption
compared with controls, and male rats treated with 1200 mg/kg/day had significantly
increased water consumption compared with those treated with 100 and 900 mg/kg/day.
Average daily food consumption in treated males and females did not differ significantly
from controls. Actual data and p values were not reported for water and food
consumption. Treated rats of all groups had diarrhea throughout the study, but it did not
appear to be more severe with increasing dose, and it was not accompanied by
histopat ho logic effects (Olson, 1992).
Average final body weight was decreased in a dose-related manner in both male and
female treated groups, but the only statistically significant (pso.05) difference was
between the female 1200 mg/kg/day and control groups. Organ weight measurements
showed significantly (p£0.05) increased relative kidney weight in females at >300
mg/kg/day, increased absolute and relative kidney weights in males at >900 mg/kg/day,
increased relative liver weight in males at £900 mg/kg/day, increased relative liver, heart
and thymus weights in females at 900 mg/kg/day, and increased absolute and relative
lung weights in males at 1200 mg/kg/day. The increases are generally dose-related but
the inconsistent pattern of statistical significance (p>0.05) for some of the organ weights
(e.g., significant changes at 900 mg/kg/day but not 1200 mg/kg/day) may be related to
smaller numbers of surviving animals at the higher doses (8 and 6 females at 900 and
1200 mg/kg/day, respectively).
MTBE.QTE -20- 08/02/93
-------
There were no distinct dose-related variations in hematology values in either sex,
although several parameters were significantly (p£0.05) altered at 1200 mg/kg/day.
These changes included decreased white blood cell count and increased erythrocyte
count, hemoglobin concentration and hematocrit in females, and decreased mean
corpuscular volume in males. The clinical chemistry evaluations showed dose-related
changes in several parameters, consisting of statistically significant decreased BUN in
both sexes, increased serum cholesterol in females and decreased serum creatine in
males at £100 mg/kg/day, and decreased serum glucose and calcium in females and
increased AST in males at £300 mg/kg/day.
Treatment-related pathologic changes were observed only in the kidneys of the male
rats treated with 1200 mg/kg/day. Chronic nephropathy, characterized by tubular
degenerative changes, was common in both the control and treated male rats. These
degenerative changes, however, were more severe in the treated male rats than in control
males. In addition, 50% (5/10) of the treated male rats had small numbers of tubules that
were plugged with granular cysts, and all of the treated males had slightly increased
numbers of cytoplasmic hyaline droplets in proximal tubular epithelial cells. These
changes appeared to be consistent with male rat hyaline droplet (o^-globulin)
nephropathy. No information on kidney histology in the lower dose groups was reported.
In a subchronic inhalation study, groups of 10 male and 10 female CD rats
(Sprague-Dawley derived) were exposed by inhalation to MTBE (purity not reported) at
MTBE.QTE -21- 08/02/93
-------
concentrations of 0, 250, 500 and 1000 ppm for 6 hours/day, 5 days/week for 13 weeks
(Greenough et al., 1980). No effects on survival, body weight, food or water
consumption, hematology, clinical chemistry or urinalysis were found following exposure.
An increasing depth of anesthesia was found with increasing exposure concentration; no
additional information regarding this effect was reported. No compound-related effects
were found following gross necropsy or histologic examination of tissues and organs from
all animals. A slight reduction in absolute and relative lung weight was found in female
rats exposed to 1000 ppm MTBE.
In another subchronic inhalation study (Dodd and Kintigh, 1989), groups of 25 male
and 25 female Fischer 344 rats were exposed to target concentrations of 0, 800, 4000
or 8000 ppm MTBE 6 hours/day, 5 days/week for 13 weeks. Evaluations included clinical
and ophthalmic observations, food consumption, body weight and hematology (five
rats/sex/group). Pathology (comprehensive organ weight and histology) was evaluated
in 15 rats/sex/group and nervous system pathology (nervous system histology, brain
weight, brain measurements) was evaluated in 6 or 10 rats/sex/group. Behavioral
evaluations (functional observational battery) were performed on 10 rats/sex/group prior
to the first exposure and at exposure weeks 1, 2, 4, 8 and 13, and motor activity was
assessed in 15 rats/sex/group prior to the first exposure and at exposure weeks 4, 8
and 13.
MTBE.QTE -22- 08/02/93
-------
Ataxia was observed immediately following exposure for the first 25 days of the
study in all of the rats exposed to 8000 ppm. No other clinical signs of toxicity were
reported for this or any other exposure group. Slight hematologic alterations occurred in
males including significantly (p<0.01) reduced mean corpuscular hemoglobin
concentration (MCHC) (week 14) and increased mean corpuscular volume (MCV) (weeks
5 and 14), mean corpuscular hemoglobin (MCH) (week 5 and week 14 at 8000 ppm) and
reticulocytes (week 14) at £4000 ppm. At 8000 ppm, the following parameters were
significantly reduced (p<0.01): leukocytes (week 5), lymphocytes (weeks 5 and 14),
hematocrit (HOT) (week 14), reticulocytes (week 14) and segmented neutrophils (week
14). Segmented neutrophils were also significantly increased in females at week 14 at
8000 ppm. The only significant (p<0.05) biochemical finding was increased cortisone
levels in both sexes at 8000 ppm. Other effects included reduced body weight gain in
both sexes during the first 1-4 weeks of exposure at £4000 ppm, concentration-related
increased relative weights of liver, kidney and adrenals in both sexes at £4000 ppm, and
increased degree but not frequency of hemosiderosis in spleen and number and/or size
of hyaline droplets in renal proximal tubules in males at 8000 ppm.
The nervous system necropsies showed significantly (p>0.05) decreased brain
length in male rats at £4000 ppm (concentration-related) and reduced absolute brain
weight in both sexes at 8000 ppm (no changes in relative brain weight or brain width were
observed). The neurobehavioral assessments showed some statistically significant
findings (e.g., elevated body temperature, decreased latency to rotate on an inclined
MTBE.QTE -23- 08/02/93
-------
findings (e.g. elevated body temperature, decreased latency to rotate on an inclined
screen, decreased hindlimb grip strength, decreased motor activity) primarily at 4000 and
8000 ppm that did not show clear dose-response relationships or occurred sporadically.
Although the methods section of the report stated that the animals were monitored during
exposure for signs of toxicity, the same "boilerplate1 was used in the methods section of
the 13-day range-finding study, which did report effects during exposure, whereas no
mention of the results of any such monitoring is made in the results section of the
subchronic study. Because the range-finding study detected signs of central nervous
system (CNS) depression in the same strain and size rats during exposure at 2000 and
4000 ppm, but did not detect neurotoxic effects in the neurobehavioral observational
battery administered after the 13th exposure to 2000 or 4000 ppm, it would appear that
observation during exposure is essential to detect the threshold for MTBE neurotoxicity,
and this monitoring during exposure may not have been performed or reported in the
•
subchronic study.
In a chronic study, four groups of CO-1 mice (50/sex) were exposed to target
concentrations of 0, 400, 3000 or 8000 ppm MTBE for 6 hours/day, 5 days/week for 18
months (Burleigh-Flayer et a I., 1992). Evaluations included clinical observations, body
and organ weights, hematologic evaluations, urinalysis, gross necropsy, and
histopathology.
MTBE.QTE -24- 07/08/93
-------
observed in both sexes in the mid- and high-dose groups, including blepharospasm
(spasmodic contraction of the orbicular muscle of the eye), hypoactivrty, ataxia, stereotypy
(3000 ppm only), prostration (8000 ppm only), and lack of startle reflex. The only clinical
effect reported in both sexes of the 8000 ppm group was ataxia. Other effects reported
in both sexes of the high-dose group included a decreased body weight gain and absolute
body weight, and a slight decrease in urinary pH. No hematologic effects were reported.
Dose-related increases in liver weight (absolute and relative to body and brain
weights) were reported in both mafe-and female mice; with only minimal effects in the 400
ppm group. Increases in kidney weight were reported for high-dose female mice and all
groups of exposed male mice, but not in a dose-related manner. Decreases in absolute
brain and spleen weight were also reported in both sexes of the high-dose group.
Histopathologic evaluation revealed no lesions in any of these organs except for the liver.
An increased incidence of hepatocellular hypertrophy was seen in males in the 3000 ppm
group and both sexes in the 8000 ppm group. The only neoplastic lesion reported was
an increased number of hepatocellular adenomas in female mice in the 8000 ppm group;
a dose-related response was not apparent.
The increased incidence of early mortality, anesthetic effects, and the significant (as
much as 24%) decrease in body weight suggests that the highest concentration of 8000
ppm exceeded the maximum tolerated dose (MTD) for this study. The lowest
MTBE.QTE -25- 08/02/93
-------
concentration (400 ppm) was a NOAEL, and the 3000 ppm group represented a LOAEL
based on clinical effects and organ weight changes in the mice.
The same laboratory performed a study using F344 rats (50/sex/group) that were
exposed to target concentrations of 0, 400, 3000 or 8000 ppm MTBE for 6 hours/day, 5
days/week for 24 months (Chun et al., 1992). Effects were more severe in the rats than
in mice, with increased mortality reported in males from both the 3000 and 8000 ppm
groups, leading to earlier sacrifice times at 97 and 82 weeks, respectively. Chronic,
progressive nephropathy was indicated to be the main cause of death in the mid- and
high-dose groups, and also contributed to a slight increase in mortality in the 400 ppm
group. Mortality and survival time for females were not significantly different between
exposed and control rats.
•
Clinical signs reported in rats exposed to 3000 and 8000 ppm included
blepharospasm, hypoactivrty, ataxia, lack of startle reflex, and swollen periocular tissue
and/or salivation (males only).
Body weight gain and absolute body weight were decreased in both sexes of the
high-dose group. No hematotogic effects were reported in any group. Dose-related
increases in kidney and liver weights (absolute and relative to body and brain weights)
were reported in females in the mid- and high-dose groups. No histopathologic findings
were reported in the liver. Increases in gross and microscopic kidney changes indicative
MTBE.QTE -26- 08/02/93
-------
of nephropathy were seen in a dose-related manner in all groups of exposed male rats
and in females exposed to 3000 and 8000 ppm MTBE.
The only neoplastic finding in rats, was an increased number of renal tubular cell
tumors in males exposed to 3000 and 8000 ppm MTBE, originally suggested to have
resulted from the accumulation of o^-globulin. Whether the etiology of these tumors can
be fully attributed to the accumulation of o^-globulin has since been questioned (Garman,
1993). This issue requires further investigation before any conclusions can be drawn.
Based on nephropathy that was reported in all groups of exposed male rats, a true
NOAEL could not be identified for this study. Both the 3000 and 8000 ppm exposures
exceeded the MTD, as evidenced by increased mortality.
REPRODUCTIVE AND DEVELOPMENTAL TOXICITY
A two-litter, one-generation inhalation reproduction study was performed in which
groups of 15 male CD rats (Sprague-Dawley derived) were exposed to actual concentra-
tions of 0,290,1180 or 2860 ppm MTBE (95-96% pure) for 6 hours/day, 5 days/week for
a premating interval of 12 weeks, and groups of 30 female rats were similarly exposed
to 0,300,1240 or 2980 ppm for a premating interval of 3 weeks (Biles et a I., 1987). The
exposures continued in the males during mating intervals. In the females, exposure
increased to 6 hours/day, 7 days/week during gestation days 0-20 and from days 5-21
of lactation following delivery of the F1a litters. The litters were not exposed. Both the
MTBE.QTE -27- 08/02/93
-------
male and female animals underwent a 2-week resting period and then produced a second
litter (FJ under the same regimen described above. In the parental animals, no adverse
effects were observed on mortality, weight gain, "in-life observations" for gross signs of
toxicity, or on the macro- or microscopic appearance of male or female reproductive
organs. In females, no effects on the pregnancy rate were found in the breeding for the
Fu litters. Slightly lower pregnancy rates were found in the breeding for the second (F1b)
litters in all treated groups, but these findings were not statistically significant or dose-
related. For males, it was reported that no adverse effects were found on reproductive
function, but it is not possible to distinguish between male and female reproductive
function when the endpoint is pregnancy rates. A nonsignificant increase in the incidence
of dilated renal pelves was found in the dams exposed to 300 and 2980 ppm, but not
1240 ppm MTBE, and hence did not appear to be related to treatment. Histologic
examination of tissues other than the testes, epididymis and ovaries was not performed.
In both the F1t and F1b generations, slightly lower body weights (not statistically
significant) were found in the pups nursing from dams exposed to 1240 and 2980 ppm
MTBE at.days 14 and 21 of lactation. Pup survival indices were significantly lower during
lactation days 0-4 in the F1b litter in the low- and middle-dose groups, but since the high-
dose group was not affected and no decrease in survival was found in the F1a litter, this
finding was not believed to be treatment-related. No abnormalities in the pups of either
litter were found following gross external or internal examination. Skeletal examinations
were not performed on the pups from either litter.
MTBE.QTE -28- 08/02/93
-------
A two-generation inhalation reproduction study was conducted in which CD
(Sprague-Dawley) rats were exposed to mean MTBE concentrations of 0, 400, 3000 and
8000 ppm (Neeper-Bradley, 1991). Groups of 25 male and 25 female P0 rats (age
approximately 6 weeks) per concentration were exposed for 10 weeks and then bred
once to produce the F, generation. Groups of 25 male and 25 female randomly selected
F! pups per concentration were exposed for at least 8 weeks and bred to produce F2
litters. In both parental generations, the exposures continued through mating, gestation
and lactation. The rats were exposed for 6 hours/day, 5 days/week prior to mating and
for 6 hours/day, 7 days/week during mating, gestation and postnatal periods. The
prebreeding exposures for the selected F, pups began after weaning (age 29-31 days).
Parental evaluations included clinical signs, food consumption, body weight, liver weight
(F, generation only), gross pathology and histology (respiratory tract, reproductive tissues
and tissues with gross lesions in control and high dose groups). Viability, survival, body
•
weight and sex distribution were evaluated in offspring.
Parental effects observed during the prebreeding exposures included hypoactivity,
lack of startle reflex, blepharospasm and increased relative liver weight (F, generation)
at £3000 ppm; and perioral wetness, ataxia, reduced food consumption during the first
2-3 weeks (P0 and F, males) and reduced body weight and body weight gain throughout
the exposure period (P0 and F, males and F, females) at 8000 ppm. Histopathologic
examinations did not reveal any treatment-related lesions in any of the three generations.
There were also no treatment-related effects on mating, fertility and gestational indices
MTBE.QTE -29- 07/08/93
-------
in either parental generation. Postnatal effects included significantly reduced body weight
and body weight gain in F1 and F2 pups (principally during the latter part of lactation
period) at £3000 ppm, and reduced F2 pup survival on postnatal day 4 (93.5% compared
to 98.1% in controls) at 8000 ppm. Thecefore, the LOAEL is the same (3000 ppm) for
parental and offspring effects.
The developmental effects of inhaled MTBE were determined in rats and mice by
Conaway et al. (1985). Groups of 25 female CD rats (Sprague-Dawley derived) and 30
female CD-1 mice were exposed to target concentrations of 0, 250, 1000 or 2500 ppm
MTBE (95-99% pure) for 6 hours/day during gestation days 6-15. Actual levels were
somewhat higher, -0,260,1100 and 3300 ppm, and there were some problems in vapor
generation (the mid- and high-exposure levels included a substantial amount of aerosol,
most of which had a particle diameter less than 1-2 \m) and with leakage during
sampling/analyzing. No treatment-related effects on mortality, overt signs of toxicrty
(checked after exposure), maternal body weight, water consumption, liver weight,
pregnancy rate, number of implants, resorptions and live fetuses, sex ratios or gross
pathology were found in either the rats or mice. Food consumption was significantly
decreased in all three groups of treated rats during gestation days 9-12 and was slightly,
but not significantly, decreased in all three groups of treated mice during gestation days
12-15. Histopathologic examinations were not performed in the dams or fetuses of the
rats or mice. No treatment-related fetal abnormalities (external, soft-tissue or skeletal
malformations) were found in rats. In mice, a slight (nonsignificant) increase in the
MTBE.QTE -30- 07/08/93
-------
incidence of fused stemebrae was found in fetuses and litters exposed to 3300 ppm
MTBE. Fetal body weights in both the rats and mice were comparable to controls.
More recent developmental toxicity studies in mice (Tyl and Neeper-Bradley, 1989)
and rabbits (Tyl, 1989) reported maternal and fetal effects at higher inhalation exposure
levels than those tested in the study by (Conaway et al., 1985). In these studies, groups
of 30 female CD-1 mice and 15 female New Zealand rabbits were exposed to target
concentrations of 0,1000,4000 or 8000 ppm MTBE for 6 hours/day on days 6-15 (mice)
or 6-18 (rabbits) of gestation. Actual monitored exposure levels were very close to target
levels (1035, 4076 and 8153 ppm in mice, and 1021, 4058 and 8021 ppm in rabbits).
The lowest target exposure level, 1000 ppm, was a NOAEL for both species. In mice,
4000 ppm produced hypoactivrty, ataxia, a decrease in maternal body weight, which was
not significant but was part of a dose-related trend, and a significant decrease in fetal
body weight and significant reduction in fetal ossification. Hypoactivrty and ataxia in this
group were observed during exposure only and not after exposure when the individual
animals were observed to determine incidence of clinical signs. The effects in mice at
8000 ppm were more severe and included not only hypoactivity and ataxia, but also
prostration, labored respiration and lacrimation, observed both during exposure and after
exposure. Additional findings at 8000 ppm were significantly reduced maternal body
weight relative to controls and reduced body weight gain and food consumption, adverse
effects on gestational indices (e.g., decreased viable implantations, increased resorptions
and dead fetuses), decreased fetal body weights and ossification and an increase in the
MTBE.QTE -31- 07/08/93
-------
incidence of cleft palate. In rabbits, 4000 ppm produced a decrease in maternal body
weight gain and food consumption. These effects were also seen at 8000 ppm, plus a
significant increase (14%) in relative liver weight that the authors suggested could be due
to enzyme induction. Additional effects at 8000 ppm were hypoactivrty and ataxia,
observed only during exposure. There were no compound-related effects on gestational
or fetal indices in the rabbits.
QUANTIFICATION OF NONCARCINOGENIC EFFECTS
The development of a quantitative risk assessment based on the toxicologic data on
MTBE following oral exposure is currently under Agency review.
CARCINOGENIC EFFECTS
HUMAN DATA
Pertinent data regarding the potential carcinogenic effects of MTBE in humans were
not located in the available literature.
LABORATORY ANIMAL DATA
Oral Studies
Pertinent data regarding the potential carcinogenic effects of MTBE in animals after
oral exposure were not located in the available literature.
MTBE.QTE -32- 08/02/93
-------
Inhalation Studies
In an inhalation bioassay, four groups of CD-1 mice and Fischer 344 rats
(50/sex/species) were exposed to target concentrations of 0, 400, 3000 or 8000 ppm
MTBE for 6 hours/day, 5 days/week for 18 months in mice (Burleigh-Flayer et al., 1992)
and for 24 months in rats (Chun et al., 1992). Evaluations included clinical observations,
body and organ weights, hematologic evaluations, urinalysis, gross necropsy, and
histopathology. These inhalation bioassays are currently (July, 1993) being reviewed by
the Human Health Assessment Group of the U.S. EPA.
Non-neoplastic findings in mice, reported earlier in this document, suggested that
400 ppm was a NOAEL for this study and 3000 ppm was the LOAEL, based on clinical
effects and body and organ weight effects. The increased incidence of early mortality,
anesthetic effects, and the significant decrease in body weight (as much as 24%) suggest
that the highest concentration of 8000 ppm exceeded the MTD for this study. The only
neoplastic lesion reported in mice was an increased number of hepatocellular adenomas
in females in the 8000 ppm group, but a dose-related response was not apparent (2/50,
1/50, 2/50 and 10/50 for control, low-, mid- and high-dose groups, respectively). In
females, only one hepatocellular carcinoma was seen at the low and high doses; none
were observed in controls or the mid-dose group. In male mice, the incidence of
hepatocellular carcinomas was statistically significantly increased in the high-dose group
when an adjustment was made for mortality (2/42, 4/45, 3/41 and 8/34 for the control,
low-, mid- and high-dose groups, respectively); the Cochran-Armitage trend test was also
MTBE.QTE -33- 08/02/93
-------
significant for carcinomas (p=0.004) (Jinot, 1993). There was no increase in the
frequency of hepatocellular adenomas in male mice, but the combined incidence of
adenomas and carcinomas was significantly increased for high-dose males, and the
Cochran-Armitage trend test on the combined tumors was significant (p=0.025) (Jinot,
1993). The investigators noted that the combined frequency of hepatic tumors in high-
dose males is within the range of historical controls for 24-month studies in CD-1 mice.
This bioassay was terminated at 18 months, however, making the validity of this
comparison questionable. Statistical analysis of the tumors in the animals that died
during the study (0/18,2/12,2/19 and 6/24 for control, low-, mid- and high-dose groups,
respectively) also demonstrated a significant difference (p=0.026) for the pairwise
comparison between controls and high-dose males, suggesting a decreased latency time
for tumors developing in mice exposed to MTBE.
•
Rats were more sensitive to MTBE, with increased mortality reported in both the
mid-dose (3000 ppm) and high-dose (8000 ppm) groups. The MTD was exceeded for
both of these concentrations. The primary neoplastic finding in rats was an increase in
the incidence of renal tubular cell tumors in males exposed to 3000 and 8000 ppm. The
investigators reported that these lesions were considered to result from the accumulation
of o^-globulin, a protein specific to male rats. The report on the anatomic pathology
study, however, indicates that the renal tumors cannot be solely attributed to the
accumulation of o^-globulin because an increased incidence of nephropathy was seen
in female rats as well as male rats (Garman, 1993). An analysis of o^-globulin in the
MTBE.QTE -34- 08/02/93
-------
13-week inhalation study (Dodd and Kintigh, 1989) also yielded equivocal results. The
percentage of renal cortex staining for o^-globulin was approximately doubled in male
rats exposed to MTBE, but the increase was not concentration-related (Garman, 1993).
It was further noted that the nephropathy present in the 24-month study was reported to
be the same for male and female rats and did not differ histologically from the
spontaneous nephropathy that is common in older rats (Garman, 1993). It was
suggested that the increased nephropathy seen in the MTBE-exposed rats was more
likely an exacerbation of the spontaneous disease rather than being a direct effect of
MTBE exposure (Garman, 1993). 'The only other neoplastic lesion reported was an
increased incidence of interstitial cell adenomas of testes in male rats. The incidence of
these adenomas was 32/50 (controls), 35/50 (400 ppm), 41/50 (3000 ppm) and 47/50
(8000 ppm). Statistical significance was demonstrated for both the 3000 and 8000 ppm
groups (p=0.035 and p=0.0002, respectively), and the Cochran-Armrtage trend test
(p=0.0001) (Jinot, 1993). The study report suggests that the increased incidences in the
3000 and 8000 ppm groups may be the consequence of an unusually low incidence in
the controls (64%, compared with 86 and 91% in controls from two other studies
conducted at the same laboratory). The strong trend, however, suggests that the
testicular adenomas do represent a treatment-related effect.
Injection Studies
Pertinent data regarding the potential carcinogenic effects of MTBE in animals after
administration by injection were not located in the available literature.
MTBE.QTE -35- 08/02/93
-------
SHORT-TERM STUDIES
No evidence of mutagenicrty was found in the Ames Salmonella/Saccharomyces
assay both with and without metabolic activation (<10 u,L/plate) (Arco Chemical Company,
1980). In a mouse lymphoma forward mutation assay, no mutagenic activity was found
without metabolic activation, but activity was found in the rat liver 89-activated assay
system (Arco Chemical Company, 1980). In the presence of S9, MTBE consistently
produced a dose-related increase in mutation frequency in four replicates using Arco
MTBE (96.2% purity) and two replicates using a commercial MTBE (99.1% purity). MTBE
did not induce sex-linked recessive lethal mutations in a feeding study using male
Drosophila melanogaster (Semau, 1989).
MTBE did not induce sister chromatid exchanges or chromosomal aberrations in
Chinese hamster ovary cells in vitro with either activated or nonactivated media (<5.0
u,L/mL) (Arco Chemical Company, 1980). No evidence of a clastogenic effect was found
in bone marrow of rats administered MTBE orally at single or repeated daily doses (for
5 days) of 0, 0.04, 0.13 and 0.4 mL/kg or mLAg/day (Arco Chemical Company, 1980).
MTBE also did not induce bone marrow chromosomal aberrations in rats that inhaled
measured concentrations of 776, 4098 or 8086 ppm for 6 hours/day on 5 consecutive
days (Vergnes and Morabit, 1989).
MTBE.QTE -36- 07/08/93
-------
QUANTIFICATION OF CARCINOGENIC EFFECTS
WEIGHT-OF-EVIDENCE FOR CLASSIFICATION
No human data are available on the carcinogenicfty of MTBE. Inhalation bioassays
have been conducted in CD-1 mice anal Fischer 344 rats; these studies are currently
(July, 1993) under Agency review. The CRAVE Work Group of the U.S. EPA has not yet
evaluated the carcinogenicity data on MTBE.
QUANTITATIVE ESTIMATE
A quantitative estimate of carcinogen icity for MTBE is not available.
EXISTING GUIDELINES, RECOMMENDATIONS AND STANDARDS
Not available at this time.
SPECIAL GROUPS AT RISK
No especially sensitive high risk human population has been identified for MTBE.
MTBE.QTE -37- 08/02/93
-------
REFERENCES
Allen, M.J., T.J. Borody, T.F. Bugliosi, G.R. May, N.F. LaRusso and J.L Thistle. 1985a.
Cholelitholysis using methyl tertiary-butyl ether. Gastroenterology. 88:122-125.
Allen, M.J., T.J. Borody, T.F. Bugliosi, G.R. May, N.F. LaRusso and J.L Thistle. 1985b.
Rapid dissolution of gallstones by methyl tertiary-butyl ether. N. Engl. J. Med. 312:
217-220.
Arco Chemical Company. 1980. Methyl tertiary-butyl ether: Acute toxicological studies.
Unpublished study from Arco Research and Development, Glenolden, PA.
Biles, R.W., R.E. Schroeder and C.E. Holdsworth. 1987. Methyl tertiary butyl ether
inhalation in rats: A single generation reproduction study. Toxicol. Ind. Health. 3:
519-534.
Bio/dynamics, Inc. 1981. A Nine-day inhalation toxicity study of MTBE in the rat. Project
No. 80-7452. Unpublished report submitted to American Petroleum Institute, Washington,
DC. 352 p.
MTBE.QTE -38- 06/18/93
-------
Bio/dynamics, Inc. 1984. The metabolic fate of methyl-t-butyl ether (MtBE) following an
acute intraperrtoneal injection. Project No. 80089. Unpublished report submitted to
American Petroleum Institute, Washington, DC. 150 p.
Bioresearch Laboratories. I990a. Pharmacokinetics of methyl tert-butyl ether (MTBE)
and tert-butyl alcohol (TBA) in male and female Rscher-344 rats after administration of
MTBE by the intravenous, oral and dermal routes. Report no. 38842. Prepared by Bushy
Run Research Center, Union Carbide Corporation, for the Methyl Tertiary Butyl Ether
Committee, Washington, DC. Microfiche No. OTS0528044.
Bioresearch Laboratories. I990b. Mass balance of radioactivity and metabolism of
methyl tert-butyl ether (MTBE) in male and female Rscher-344 rats after intravenous, oral
and dermal administration of 14MTBE. Report no. 38843. Prepared by Bushy Run
•
Research Center, Union Carbide Corporation, for the Methyl Tertiary Butyl Ether
Committee, Washington, DC. Microfiche No. OTS0528044.
Bruckstein, A.M. 1990. Nonsurgical management of cholelithiasis. Arch. Intern. Med.
150: 960-964.
Burleigh-Rayer, H.O., J.S. Chun and W.J. Kintigh. 1992. Methyl tertiary butyl ether:
Vapor inhalation oncogenictty study in CD-1 mice. Bushy Run Research Center, Export,
PA. Laboratory Project ID# 91N0013A.
MTBE.QTE -39- 06/18/93
-------
Chun, J.S., H.D. Burleigh-Flayer and W.J. Kintigh. 1992. Methyl tertiary butyl ether:
Vapor inhalation oncogenicity study in Fischer 344 rats. Bushy Run Research Center,
Export, PA. Laboratory Project ID# 91N0013B.
•
Conaway, C.C., R.E. Schroeder and N.K. Snyder. 1985. Teratology evaluation of methyl
tertiary-butyl ether in rats and mice. J. Toxicol. Environ. Health. 16: 797-809.
Daubert, T.E. and R.P. Danner. 1992. Physical and Thermodynamic Properties of Pure
Chemicals: Data Compilation. Part I. Design Institute for Physical Property Data.
American Institute of Chemical Engineers. Hemispheric Publishing Corp., Washington,
D.C.
Dodd, D.E. and W.J. Kintigh. 1989. Methyl tertiary butyl ether (MTBE): Repeated
(13-week) vapor inhalation study in rats with neurotoxicrty evaluation (unpublished study).
Union Carbide, Bushy Run Research Center for MTBE Committee. TSCATS 403189.
EPA/OTS #FYI-OTS-0889-0689.
Garman, R.H. 1993. DVM Consultants in Veterinary Pathology, Murrysville, PA. Letter
to LS. Andrews, ARCO Chemical Co., Newtown Square, PA. May 3.
Gonzaga, R.A.F., F.R. Lima and S. Carneiro. 1985. Dissolution of gallstones by methyl
tertiary-butyl ether. N. Engl. J. Med. 313: 385.
MTBE.QTE -40- 06/18/93
-------
Goth, A. 1974. Medical Pharmacology: Principles and Concepts, 7th ed. C.V. Mosby
Company, St. Louis, MO.
Greenough, R.J., P. McDonald, P. Robinson et al. 1980. Methyl Tertiary-Butyl Ether
(Driveron) Three Month Inhalation Toxictty in Rats. Project No. 413038. Unpublished
report submitted to Chemische Werke Hols AG, Marl, West Germany. 230 p.
Hawley, G.G., Ed. 1981. The Condensed Chemical Dictionary, 10th ed. Van Nostrand
Reinhold Co, New York, NY. p. 761-772.
Jinot, J. 1993. Statistical analysis of tumors from MTBE studies. Internal memorandum
to J. Parker, Chief, Carcinogen Assessment Toxicology Branch, U.S. EPA, Washington
DC. May 17.
Juliani, G., G. Gandini, S. Gabasio, L Bonardi, E. Fascetti and L Gremo. 1985.
Colelitolisi chimica transcutanea con metil-ter-butil etere (MTBE). Radiol. Med. (Torino)
71:569-574. (Hal.)
LaGrone, F.C. 1991. Potential community exposure to toxic chemicals. Environ. Sci.
Technol. 25:366-368.
NAS (National Academy of Science). 1977. Drinking Water and Health. Vol. 1. p. 19-63.
MTBE.QTE -41- 06/18/93
-------
MAS (National Academy of Science). 1980. Drinking Water and Health. Vol.3, p. 25-67.
Neeper-Bradley, T.L 1991. Two-generation reproduction study of inhaled methyl tert-
butyl ether in CD Sprague-Dawley rats (unpublished study). Union Carbide, Bushy Run
•
Research Center.
Olson, G.R. 1992. Pathologist, Pathology Associates, Inc. Personal Communication to
S.F. Velazquez, U.S. EPA, Cincinnati, OH. September 30.
Ponchon, T., J. Baroud, B. Pujol, P.J. Valette and D. Perrot. 1988. Renal failure during
dissolution of gallstone by methyl tert-butyl ether. Lancet 2: 276-277.
Robinson, M., R.H. Bruner and G.R. Olson. 1990. Fourteen-day and ninety-day oral
toxicity studies of methyl-tert-butyl ether in Sprague-Dawley rats. J. Am. Coll. Toxicol.
9(5): 525-540.
Savolainen, H., P. Pfaffli and E. Elovaara. 1985. Biochemical effects of methyl tertiary-
butyl ether in extended vapour exposure in rats. Arch. Toxicol. 57: 285-288.
MTBE.QTE -42- 06/18/93
-------
Sernau, R.C. 1989. Mutagenicity test on methyl tertiary butyl ether Drosophila
rnelanogaster sex-linked recessive lethal test. Prepared by Hazelton Laboratories
America, Inc., for the Methyl Tertiary Butyl Ether Committee, Washington, DC. Microfiche
No. OTS0528039.
Tyl, R.W. 1989. Developmental toxicity study of inhaled methyl tertiary butyl ether in New
Zealand white rabbits. Project Report 51-268. Prepared by Bushy Run Research Center,
Union Carbide Corporation for the Methyl Tertiary Butyl Ether Committee, Washington,
DC. Microfiche No. OTS0000689-1. (Cited as Union Carbide, 1989b in U.S. EPA, 1993)
Tyl, R.W. and T.L Neeper-Bradley. 1989. Developmental toxicity study of inhaled methyl
tertiary butyl ether in CD-1 mice. Project Report 52-526. Prepared by Bushy Run
Research Center, Union Carbide Corporation for the Methyl Tertiary Butyl Ether
•
Committee, Washington, DC. Microfiche No. OTS0000689-1. (Cited as Union Carbide,
1989a in U.S. EPA, 1993)
U.S. EPA. 1980. Guidelines and Methodology Used in the Preparation of Health Effect
Assessment Chapters of the Consent Decree Water Criteria Documents. Federal
Register. 45 (231): 79347-79357.
U.S. EPA. 1986. Guidelines for Carcinogen Risk Assessment Federal Register.
51 (185): 33992-34003.
MTBE.QTE -43- 06/18/93
-------
U.S. EPA. 1987. Information on the Incidence of Methyl tert-Butyl Ether in Groundwater
from Different States. Submitted to OTS. Microfiche No. OTS 0000574-0. Document No.
FYI-OTS-0987-0574.
U.S. EPA. 1991. Alphas-globulin: Association with Chemically Induced Renal Toxicrty
and Neoplasm in the Male Rat. Prepared for the Risk Assessment Forum, U.S. EPA,
Washington, DC. EPA/625/3-91/019F.
U.S. EPA. 1993. Integrated Risk Information System (IRIS). Online. Office of Health and
Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati, OH.
Vergnes, J.S. and E.R. Morabit. 1989. Methyl tertiary butyl ether repeated exposure
vapor inhalation study in rats: In vivo cytogenetic evaluation. Project No. 51-635.
•
Prepared by Bushy Run Research Center, Union Carbide Corporation for the Methyl
Tertiary Butyl Ether Committee, Washington, DC. Microfiche No. OTS0528040.
Weast, R.C., Ed. 1985. CRC Handbook of Chemistry and Physics, 66th ed. CRC Press,
Boca Raton, FL p. C-177.
Wyngaarden, J.B. 1986. New nonsurgicaJ treatment removes gallstones. J. Am. Med.
Assoc. 256: 1692.
MTBE.QTE -44- 06/18/93
-------
Yalkowsky, S.H. 1989. Arizona Database of Aqueous Solubility. College of Pharmacy,
University of Arizona, Tucson, AZ.
MTBE.QTE -45- 06/18/93
------- |